The present invention relates to scanning funneling microscopes which by means of optically induced spin polarization measure the magnetic properties of surfaces.
Scanning tunneling microscopy per se is well known in the art to permit the inspection of surface topographies down to the atomic level, based on the strong dependency of the tunneling current upon the distance of the tunnel tip from the surface being investigated. A summary of the features of scanning tunneling microscopy is provided by the inventors of the Scanning Tunneling Microscope, G. Binnig and H. Rohrer, "Scanning Tunneling Microscopy", IBM Journal of Research and Development, Vol. 30, No. 4, July 1986, pp. 355-369, where a useful catalogue of relevant literature published prior to 1986 is provided.
As is well known, the tip of a scanning tunneling microscope closely follows the profile of the surface under investigation by monitoring the tunneling current and providing a feedback signal dependent on the deviation of the distance between the tip and surface from a predetermined value. By registering the feedback signal with the coordinates of the tip at the time of registration, an image of the topography of the surface is generated. The resolution obtainable is on the order of the size of a single atom.
Qualitative information at comparable resolution, i.e. information regarding the chemical elements present at (or near) the surface of a specimen may be obtained from a field-emission scanning Auger electron microscope of the type described in EP-A-1-0 189 498. The energy of the Auger electrons emitted by a material hit by a sharply focused field-emitted electron beam is characteristic of the emitting element.
Neither the scanning tunneling microscope nor the Auger electron microscope can provide information on the magnetic properties of the surface of a specimen. And no other instrument is known today which would permit the detection of magnetic phenomena on a surface with a resolution close to the atomic level.
In connection with a search for storage devices having higher packing densities, it has been proposed in IBM Technical Disclosure Bulletin, Vol. 30, No. 4, September 1986, p. 1858, to employ a tunnel tip operated in the spin-polarized mode. In this proposal, the spin-polarization is obtained through the use of a tungsten tip having a thin layer of a ferromagnetic semiconductor such as EuS or EuO, coated thereon. The cryogenic environment required by these materials can be avoided by employing a half-metallic ferromagnet, such as a Heusler alloy, NiMnSb or PtMnSb, or CrO.sub.2, in accordance with the teaching in IBM Technical Disclosure Bulletin, Vol. 30, No. 7, December 1987, p 140.
Field-emission of spin-polarized electrons is further described in M. Landolt and Y. Yafet, "Spin-Polarization of Electrons Field Emitted from Single-Crystal Iron Surfaces", Phys. Rev. Lett. Vol. 40, No. 21, 22 May 1978, pp. 1401-1403. This paper in particular deals with the preparation and characteristics of single-crystal iron tips. However, the tips that can be prepared with the process described in this paper, have a radius approximately 10 times too large at its apex to be useful in connection with the present invention.